One of the existing techniques of identifying optical channels in an optical network is modulating each optical channel with a respective low frequency pilot tone, or dither tone, or a combination of dither/pilot tones, which uniquely identify the optical channel in the optical network. Such unique combinations of dither tones have been referred to as a channel signature or identification tag in an optical network. The channel signature is detected at various points within an optical network using signal processing methods that extract the low frequency dither tones from the background payload noise.
Methods and apparatus for inserting and detecting such dither tones, selecting their frequencies, and application in an optical network for identifying optical channels are described in the following patents issued to the applicant, all of which are incorporated herein by reference:
U.S. Pat. No. 6,968,131 “Topology discovery in optical WDM networks”;
U.S. Pat. No. 7,031,606 “Method and system for monitoring performance of optical network”;
U.S. Pat. No. 7,054,556 “Channel identification in communications networks”;
U.S. Pat. No. 7,127,165 “Method and system for compensating for side effects of cross gain modulation in amplified optical networks”;
U.S. Pat. No. 7,142,783 “Method and system for identification of channels in an optical network”; and
U.S. Pat. No. 7,184,660 “Method and system for monitoring performance of optical network”, which is a Continuation-in-Part of the U.S. Pat. No. 7,031,606.
Each optical channel is used to transmit a data stream with a given protocol, e.g. SONET, Fiber Channel, Gigabit Ethernet (GbE). The power spectrum of each optical channel data stream includes random and deterministic components, resulting in a payload spectrum that is a combination of random noise, together with fixed line components. For example, the spectrum of a SONET payload will include a random component from the scrambled payload, and a line component at multiples of 8 kHz from the repetitive SONET framing bytes. The payload spectrum (both random and line) appears as noise to the low frequency dither tone detection system. In addition, other sources of interference (for instance, power supply, or on-board clocks coupled into the detection circuit) also degrade the ability of the detection system to correctly decode the low frequency dither information. All these interference sources are collectively referred to as “noise” to the low frequency dither signal.
FIG. 1 illustrates a typical payload data spectrum 10 showing spectral density over a frequency range of about 10 kHz to 1 MHz: a random noise floor 12, a low frequency dither tone (pilot tone) 14, and some “noise” tones 16 in an optical channel of an optical network.
The purpose of the low frequency dither tone 14 may be to identify the optical channel in the network (as well as other physical network properties such as fiber identification etc.) by detecting the tone at various locations in the network, as described for example in U.S. Pat. No. 7,184,660 referenced herein.
The amplitude of the low frequency dither tone 14 is generally designed to be higher than the random noise floor 12, to be detected for example by averaging of FFT results as described in U.S. Pat. No. 7,054,556 referenced herein.
However, the presence of the “noise” tones 16 in the spectrum makes correct identification of the low frequency dither tone 14 (which in this example is an intended identification tag) more difficult, because the “noise” tones 16 could also be detected and interpreted as identification tags.
U.S. Pat. No. 7,127,165 referenced herein, for example, describes a method of rejecting noise tones resulting from cross gain modulation by dynamic thresholding of detected tones in order to separate the actual valid tones of the expected channels from the unwanted signals.
The problem of noise tones is illustrated in more detail in FIG. 2.
FIG. 2 illustrates a scenario 20 of a section of an optical network, including two optical add-drop multiplexers OADM1 and OADM2, joined by a multi-wavelength optical fiber 22. At the OADM1, a first optical channel 24 is generated at a wavelength of λ1 and modulated (encoded) in an optical modulator 26 with a first low frequency dither tone (pilot tone) 28 at a frequency of f1. A first spectrum 30 illustrates the spectral density found on the first optical channel 24 that includes a noise floor and the first pilot tone 28 at frequency f1 carried by the wavelength λ1.
The OADM1 further receives from another part of the optical network a second optical channel 32 at a wavelength λ2, which carries and is identified by a second low frequency dither tone (pilot tone) 34 at a frequency of f2. A second spectrum 36 illustrates the spectral density found on the second optical channel 32 that includes a noise floor and the second pilot tone 34 at frequency f2 carried by the wavelength λ2. In the present example, the wavelength λ2 also carries a noise tone 38 that happens to have a frequency of f1, as shown in the second spectrum 36.
The two optical channels 24 and 32 at the wavelengths λ1 and λ2 respectively, and carrying the first and second pilot tones 28 and 34 (f1 and f2) respectively, as well as the noise, are combined in the OADM1 to be sent to the OADM2 over the multi-wavelength optical fiber 22. A third spectrum 40 illustrates symbolically the spectral density that would be found on the multi-wavelength optical fiber 22, that is the superposition of the first and second spectra 30 and 36. The noise tone 38 and the first pilot tone 28 would be indistinguishable: the noise tone 38 carried by λ2 (the second optical channel 32) “hits” the first pilot tone 28 at the frequency f1.
A problem that could result from the hit of the first pilot tone 28 by the noise tone 38 is the following: the detection of the pilot tones at various locations in the network is used to maintain the network and, for example, detect the failure or misrouting of optical channels. At the OADM2 in FIG. 2, a pilot tone at the frequency f1 could be detected, even if the first optical channel 24 (carrying its pilot tone 28 at frequency f1) should fail—the noise tone 38, also at the frequency f1 could be mistaken for the first pilot tone 28, and the first optical channel 24 would appear to be working and received at the OADM2, when in fact it had failed. Even when the first optical channel 24 is working (carrying its pilot tone 28 at the frequency f1), the ‘hitting’ of the pilot tone 28 by the noise tone 38 will result in constructive and destructive interference on the tone at f1. Hence detection of the pilot tone 28 becomes erratic.
While the scenario described in FIG. 2 is simplified, it will be appreciated that a real optical network may comprise many nodes and optical (wavelength) channels, each optical channel carrying two or more low frequency dither tones, as described in the U.S. patents referenced earlier, and a considerable number of “noise” tones may occur. In such networks, noise tones which are “ghost” tones may be produced by the cross gain modulation of dither tones as described in U.S. Pat. No. 7,127,165.
While noise tones may originate inadvertently as a result of the payload modulation of an optical channel or as “ghost” tones, another source of “noise” tones may be pilot tones intentionally modulated onto an optical channel by external equipment that is connected to the network. This is illustrated in FIG. 3.
FIG. 3 shows an example of a network hierarchy 50. The network hierarchy 50 comprises an optical carrier network 52 with an optical subscriber managed sub-network 54 connected to the optical carrier network 52 through an input optical link 56 and an output optical link 58.
The optical carrier network comprises first and second optical add-drop multiplexers (OADM) 60 and 62, linked by a multi-wavelength link 64. The optical carrier network 52 may include a number of nodes such as OADMs or other optical nodes, linked in a predetermined configuration through optical links which, in general, are multi-wavelength or wavelength-division-multiplexed (WDM) links. The optical carrier network 52 illustrated in FIG. 3 is simplified and shows only the two OADM nodes 60 and 62, and a subset of its links, in order to more clearly illustrate the problem. Each WDM link transmits signals at one or more wavelengths, commonly referred to as optical wavelength channels (or simply “channels”), while an OADM is able to separate the channels carried in a link, and to combine channels into links as set up by the network control system.
In the example shown in FIG. 3, only two channels, a λ1 channel and a λ2 channel, are shown even though it will be appreciated that in general, a network comprises many channels, and each OADM routes such channels.
The first OADM 60 receives the λ2 channel over a link 66 and the λ1 channel over the input optical link 56 from the subscriber network 54. The first OADM 60 then passes both the λ1 and λ2 channels to the second OADM 62 over the multi-wavelength link 64.
The second OADM 62, having received both the λ1 and λ2 channels over the multi-wavelength link 64, demultiplexes the channels and sends the λ2 channel out to another node (not shown) over a link 68, and the λ1 channel over the output optical link 58 to the subscriber network 54.
As described before with regard to FIG. 2, the λ2 channel is tagged with a pilot tone of a frequency f2 as schematically shown in a stylized spectrum 70. Similarly, the λ1 channel is tagged within the optical carrier network 52 with a pilot tone of a frequency f1 as shown in a stylized spectrum 72. However, the λ1 channel arriving in the input optical link 56 may already be tagged with a dither tone of a frequency fx in the sub-network 54 as shown in a stylized spectrum 74. Clearly there is a potential for conflicting or overlapping dither tone assignments, and the dither tone at the frequency fx may be considered to be a “noise” tone within the optical carrier network 52. As shown in a stylized spectrum 76, all three dither tones (f1, f2, and fx) may be observed in the multi-wavelength link 64. After demultiplexing in the OADM 62, the λ2 channel may be passed to another node (not shown) in the WDM link 68, and the λ1 channel is transmitted to the sub-network 54 in the output optical link 58. The λ1 channel still carries the f1 and fx dither tones as schematically shown in a stylized spectrum 78. The presence of fx in the λ1 channel may be of importance within the sub-network 54 for confirming the continuity of the λ1 channel.
As illustrated in the foregoing, dither tones are of importance for identifying wavelength channels, but the presence of “noise” tones may hide or degrade the dither tones and interfere with their performance. Accordingly, an improved method and system for dealing with noise tones is required to alleviate these problems.